1. Field of the Invention
The present invention relates to a method of manufacturing an on-demand type print head suitable for use on an ink-jet printer for printing characters on a recording sheet of paper with a liquid ink.
2. Description of the Related Art
A first prior art print head for an ink-jet printer, disclosed in Japanese Patent Laid-open (Kokai) No. 55-86767 will be described with reference to FIGS. 8 to 10. As shown in FIG. 8, this prior art print head has an inner plate 30 and outer plates 31 attached adhesively to the opposite surfaces of the inner plate 30, respectively. As shown in FIG. 9, an ink pool 32 for storing the ink, a plurality of pressure pits 33 connected to the ink pool 32, a plurality of passages 34 extending respectively from the pressure pits 33, and a plurality of nozzles 35 connected respectively to the extremities of the passages 34 are formed in each of the opposite surfaces of the inner plate 30. The nozzles 35 are staggered on the opposite surfaces of the inner plate 30 as shown in FIG. 8 and open in one end surface of the inner plate 30 as shown in FIG. 10. As shown in FIG. 8, piezoelectric elements 36 are joined to the outer surfaces of the outer plates 31 at positions corresponding to the pressure pits 33, respectively. Voltage is applied selectively to the piezoelectric elements 36 to pressurize the corresponding pressure pits 33 to jet the ink through the corresponding nozzles 35. Since the nozzles 35 are arranged at a small pitch in two rows, the number of the nozzles 35 may be considerably large and the nozzles 35 can be arranged in a high density. However, since the nozzles 35 are connected to the wide pressure pits 33 by the curved passages 34, the pressure pits 33 and the passages 34 occupy a large space on the inner plate 30 and the number of the nozzles 35 is limited by the space available for arranging the pressure pits 33 and the passages 34.
Referring to FIG. 11 showing a second prior art print head disclosed in Japanese Patent Laid-open (Kokai) No. 63-252750, pairs of walls 39 and 40 formed by machining a pair of piezoelectric ceramic plates 44 and 45 are sandwiched between glass plates 37 and 38 so as to form pressure chambers 41 between the adjacent pairs of walls 39 and 40. A nozzle 42 is formed at one end of each pressure chamber 41 and electrodes 43 are attached to the side surfaces of the walls 39 and 40 facing the pressure chambers 41. The internal pressure of a specified pressure chamber 41 is increased by straining a specified pair of walls 39 and 40 by applying a voltage to a specified electrode 43 and grounding the electrodes 43 respectively on the opposite sides of the specified electrode 43 to jet the ink contained in the specified pressure chamber 41 through the nozzle 42.
When fabricating this print head, the piezoelectric ceramic plate 44 is attached to the glass plate 37, the piezoelectric ceramic plate 45 is attached to the glass plate 38, a plurality of grooves are cut in the piezoelectric ceramic plates 44 and 45 by a profile cutting disk to form the walls 39 and 40 on the opposite sides of each groove, and the two piezoelectric ceramic plates 44 and 45 are fixedly joined together with the corresponding walls 39 and 40 joined together end-to-end.
Referring to FIG. 12 showing a third prior art print head disclosed in Japanese Patent Laid-open (Kokai) No. 2-150355, a plurality of parallel grooves 47 are formed in a bottom plate 46 formed of a piezoelectric material and polarized in the direction of the arrows to form side walls 48 and a bottom wall 49. A top plate 51 is attached adhesively to the end surfaces 50 of the side walls 48 with an adhesive 52 to form pressure chambers. An electrode 53 is formed in a substantially half section of each side surface of each side wall 48 on the side of the top plate 51 by evaporation. Then, an ink supply port to be connected to an ink supply unit is formed in one end of each pressure chamber and an ink jet is formed in the other end of the pressure chamber to complete the print head.
When voltages of opposite polarities are applied respectively to the electrodes 53 on the two adjacent side walls 48, the adjacent side walls 48 are strained as indicated by dotted lines in FIG. 12 to increase the internal pressure of the pressure chamber formed between the adjacent side walls 48 and, consequently, the ink contained in the same pressure chamber is jetted through the ink jet.
Although the nozzles 35 of the first prior art print head disclosed in Japanese Patent Laid-open (Kokai) No. 55-86767 shown in FIGS. 8 to 10 can be arranged in a high density, pressure loss occurs in the passages 34 connecting the nozzle 35 to the wide pressure chamber 33 and hence the printer is unable to jet the ink efficiently. Since the passages 34 have different shapes respectively, different degrees of pressure loss occur respectively in the passages 34 and hence the nozzles 35 differ from each other in ink jetting performance. Such disadvantages become significant when the number of the nozzles 35 is increased. Therefore, the number of the nozzles cannot be increased unlimitedly.
In the second prior art print head disclosed in Japanese Patent Laid-open (Kokai) No. 63-252750 shown in FIG. 11, the density of the nozzles 42 is on the order of eight nozzles in 1 mm. Therefore, if the nozzles 42 are arranged in a single row it is impossible to print characters in high print quality and satisfactory resolution. Furthermore, since the end surfaces of the corresponding side walls 39 and 40 need to be aligned in joining together the two piezoelectric ceramic plates 44 and 45, the fabrication of the print head requires difficult work. When forming the electrodes 43 by an evaporation process, metal particles evaporated from an evaporation source are deposited easily on surfaces directly facing the evaporation source and the metal particles can not be easily deposited on surfaces extending obliquely to the evaporation source. Accordingly, the metal particles can be easily deposited on the end surfaces of the side walls 39 and 40 directly facing the evaporation source and it is difficult to deposit the metal particles on the side surfaces of the side walls 39 and 40 not facing the evaporation source and pinholes are liable to be formed in the electrodes 43. Therefore, it is impossible to apply an electric field uniformly to the piezoelectric ceramic plates 44 and 45. If pinholes are formed in the electrodes 43, the ink penetrates the electrodes 43 through the pinholes to corrode the piezoelectric ceramic plates 44 and 45. Still further, the evaporation process needs an expensive vacuum evaporation system and hence the electrodes 43 are costly.
In the third prior art print head disclosed in Japanese Patent Laid-open (Kokai) No. 2-150355 shown in FIG. 12, a stress to distort the side wall 48 is induced only in the upper portion of the side wall 48, and the lower portion of the side wall 48 not provided with the electrode 53 resists the distortion of the upper portion of the side wall 48. Since the side wall 48 is formed of a piezoelectric material having a high rigidity, the resistance of the lower portion of the side wall 48 makes the distortion of the upper portion of the side wall 48 further difficult, which deteriorates the ink jetting performance of the print head. Such a problem may be solved by applying a very high voltage to the electrodes 53 or by forming the side walls 48 in a very large height. However, application of a very high voltage to the electrodes 53 deteriorates the polarization of the side walls 48 and machining cost increases when the height of the side walls 48 is increased.